Objectives: Several studies have addressed the association between changes in estrogen and progesterone levels and changes in parameters of periodontitis. The purpose of this project is to investigate the mechanisms by which estrogen influence structure and function of the periodontal ligament by affecting the properties of periodontal ligament cells (PDL cells). Methods: PDL cells were obtained from teeth extracted for orthodontic reasons. The cells were cultured from periodontal tissue explants and used in passages 3-5. Estrogen (ER) expression was investigated by immunocytochemistry. Subcellular distribution of ERβ was determined using mitotracker. Expression of mitochondrial proteins was done using Western blotting. DNA and collagen synthesis was measured using incorporation of radioactive isotopes. Cytokine expression was investigated using ELISA. Results: ERβ but not ERα immunoreactivity was observed in the PDL cells. Preliminary results show that ERβ is distributed not only in the nucleus but also in the mitochondria. To study the effect of mitochondrial ERβ we will investigate synthesis of mitochondrial proteins. Estrogen increased DNA synthesis in human breast cancer cells but had no effect on PDL cell DNA or collagen synthesis. Conclusion: Human PDL cells express ERβ suggesting that estrogen affects PDL cellular function via this ER subtype. Estrogen has no effect on DNA and collagen synthesis, showing that estrogen has no beneficial effect on the periodontium via this mechanism. Instead estrogen via ERβ acts on the periodontium via another still unknown mechanism, perhaps via mitochondrial function or regulatory effect on cytokines.